This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-282852, filed Sep. 19, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor laser device and particularly to a semiconductor laser device including a semiconductor laser element adapted for an optical pickup based on a tracking servo system using a three-beam method.
2. Description of Related Art
A CD-ROM (Compact Disk-Read Only Memory) has come to be indispensable as a recording medium for a personal computer (PC). In an optical pickup used for an optical disk, tracking control generally adopts a system called a 3-beam method.
FIG. 9 shows a schematic view of an optical system adopted to the 3-beam system. A laser beam 601 emitted from a semiconductor laser element such as a laser diode (not shown) is guided to a diffraction grating 602. The diffraction grating 602 generates diffraction lights of 0-order, 1-order, and xe2x88x921-order. The diffraction lights pass through a collimator lens 603, a half-mirror 604, and an objective lens 605 and have a focus point on an optical disk 610. That is, three beams of a main beam 606 and sub-beams 607 and 608 are focused on the optical disk 610. Reflection lights from the optical disk 610 are guided to the half-mirror 604 passing again through the objective lens 605 and are reflected on the half-mirror 604, to enter into patterned PDS (photo diodes) 611. Incident light is photoelectrically converted at each PD 611. The signal outputted from each PD 611 is calculated to obtain a position shift of the beam. The calculation result is fed back to a drive part of the optical pickup and is controlled so as to let the main beam 606 follow the track 609.
The tracking control based on the 3-beam method covers a wide following range, and no limitation is put on the density and phase of the disk. Therefore, the tracking control is less influenced by variations in the disk quality. Accordingly, this is suitable for an optical pickup for reading. This method, however, has a problem of returning light from sub-beams.
That is, as shown in FIG. 10, two sub-beams 705 reflected from the disk partially return to the upper and lower sides of the laser diode chip (hereinafter called a LD chip) 701 (this is hereinafter called 3-beam returning light). The two sub-beams 705 are distant from the laser beam 703 by a distance d. For example, when the 3-beam returning light 705 is guided to the side of the sub-mount, the beams are reflected again on the side surface of the sub mount 704. Therefore, reflection light 707 is generated from the 3-beam returning light and is mixed into the optical system. Consequently, a tracking error is caused in some cases.
To avoid this problem, a sub-mount 801 as shown in FIG. 11 is used. A side surface of this sub-mount 801 that is positioned just below the laser beam emission facet of the LD chip 701 has three parts. That is, an upper part of the side surface of the sub-mount is formed to be vertical to the upper surface 802 of the sub-mount. A part of the side surface near a position at a distance d (see FIG. 10) from the emission point is inclined at an angle xcex8. A lower part of the side surface of the sub-mount is also formed to be vertical to the upper surface 802 of the sub-mount. The beam direction of the 3-beam returning light 705 is refracted in correspondence with the inclination angle xcex8 of the inclined part. For example, the beam direction of the returning light 705 which returns in a direction vertical to the laser beam emission side surface 808 is refracted by 2xcex8, according to Snell""s law. NA (Numerical Aperture) of a collimator lens for a CD is about 0.1. An estimated half angle is about 5.7 degrees where the relationship of NA=nxc3x97sin xcex8. At this time, if the inclination angle is 3 degrees or more, the returning light reflected on the side surface of the sub-mount is incident again to the collimator lens 603 (see FIG. 9), and is thus prevented from mixing into the optical system. Thus, influences from the 3-beam returning light can be eliminated by inclining the side surface of the sub mount 801 at an angle corresponding to the NA of the collimator lens.
If a countermeasure is taken only against the returning light of the 3 beam optical system, the entire side surface of the sub-mount may be inclined. However, in case of die-bonding the sub-mount to a metal heatsink or die-bonding the LD chip to the sub-mount, the optimal axis direction must be set precisely. Therefore, operation for letting the laser beam emission facet of the LD chip collide with a positioning pin is required. Hence, constant areas on the upper and lower parts of the side surface of the sub-mount are formed to be vertical to the upper surface of the sub-mount. That is, an inclined part is formed only on a part of the side surface of the sub-mount upon requirements, as shown in FIG. 11.
The sub-mount 801 is formed by a dicing process as shown in FIGS. 12A and 12B. At first, as shown in FIG. 12A, a sub-mount substrate 902 is partially cut by a blade 901 having a substantial V-shape. In this manner, a vertical part and an inclined part at an upper part of a side surface of the sub-mount are formed. Next, as shown in FIG. 12B, the sub-mount substrate 902 is cut and divided by a blade 903 having a normal shape. At this time, another vertical part at a lower part of the side surface of the sub-mount in the laser-beam emission side is formed. Thus, a sub-mount 904 having three side surfaces corresponding to the laser-beam emission facet is formed. That is, this sub-mount 904 has two flat parts respectively having heights a and e for abutting with a pin, and an inclined part as a countermeasure against 3-beam returning light, which has a height b and a depth c. The heights a and b and the depth c need to be highly precise, within a margin of error of about 10 xcexcm. Therefore, when cutting is carried out by the V-shape blade 901, the position of the blade must be controlled precisely in the plane direction and in the depth direction. In addition, when cutting is carried out by the blade 903 having a normal shape, the blade must be positioned precisely. Further, the shape of the V-shape blade changes due to friction as cutting continues. To cope with the effect of friction, the cutting depth must be changed, and the inclined part must be matched with the required dimensions. Therefore, a complicated adjustment operation is required.
As described above, the sub-mount having the structure shown in FIG. 11 has a very complicated structure and is difficult to manufacture. The cost for the sub-mount is too high to enable an effective entire cost reduction for the semiconductor laser device.
Hence, there has been a demand for a semiconductor laser device and a method for manufacturing the same, which are capable of consistently removing the influences of the returning light of three beams, and enable excellent mass-productivity.
According to an aspect of the invention, there is provided a semiconductor laser device comprising: a semiconductor laser chip having an emission facet for emitting a laser beam; and a sub-mount having a first surface on which the semiconductor laser chip is provides, and at least one second surface vertical to the first surface, wherein one of the second surface, which is arranged in line with the emission facet of the semiconductor chip, is inclined at an angle of 3 to 30 degrees to the emission facet, and the second surface which is inclined reflects reflection light of a sub-beam diffracted from the laser beam emitted from the semiconductor laser chip.